1. Field of the Invention
The invention generally relates to optical communication systems, and, in particular, to the use of three-wave optical parametric amplification for optical signal amplification in fiber-optic communication systems.
2. Description of the Prior Artxe2x80x94General
Owing to large frequency-bandwidth capacity, which is characteristic to both optical fiber and optical signals, fiber-optic technology provides the means for transmitting large volumes of information at very high speeds (high-bit-rate data transmission). The current generation of fiber-optic (lightwave) communication systems is based on the use of optical amplification and wavelength-division multiplexing (WDM). Optical amplification is used to increase signal propagation distance, which, otherwise, would be limited by optical losses in a transmitting fiber. WDM is used to increase the number of communication channels transmitted through the fiber, to increase the bit rate. In WDM systems each communication channel is assigned its own frequency band, characterized by a unique carrier frequency and spectral width. The bit rate of the data transmitted through a single fiber is thus proportional to the number of channels created in this fiber.
Two main factors that limit the number of channels are the spectral bandwidth provided by current optical amplifiers and the ultimate spectral separation between channels. The channel density cannot be infinitely high due to findamental limitations arising from nonlinear effects in the fiber and the signal modulation bandwidths used to encode the transmitted data. Current channel spacing is 50-100 GHz (corresponding to 0.4-0.8 nm at a 1550 nm communication window) and is not expected to be significantly improved beyond this level. The maximum potential bandwidth of Erbium doped fiber amplifiers, which are currently standard in communication systems, is only approximately 80 nm, limited by active-ion and glass-host spectroscopic properties. This sets the maximum limit of WDM channels potentially available in currently existing communication systems to 100-200.
However, ideally, one would wish to utilize the full spectral bandwidth available in the fiber in order to achieve the maximum number of WDM channels and, consequently, the highest bit rates in fiber communication systems. This available bandwidth usually corresponds to the spectral region where the optical attenuation in the optical fiber is at its minimum. For currently standard silica-based fibers this comprises the wavelength range between approximately 1.2 to 1.7 xcexcm. This broad bandwidth is unattainable with any known rare-earth doped fiber amplifier, including Er-doped fiber amplifiers. This is illustrated in FIG. 1, where the loss curve for silica fibers and approximate wavelength ranges for the gain spectra of the known rare-earth fibers are shown.
In optical fibers based on other types of optical glasses (eg. ZBLAxe2x80x94zirconium-barium-lanthanum-aluminum fluoride glass) optical losses can be significantly smaller, and spectral bandwidths broader, than in silica fibers, as shown in FIG. 2. The absence of suitable optical amplification sources is one of the main obstacles to the use of these broad-bandwidth and low-loss fibers in communication systems.
New types of optical amplifiers are necessary for optical communication systems, to enable full access to the broad spectral bandwidths available in optical fibers.
The ability to change the wavelength of a transmitted optical signal is a necessary function for WDM communication systems. WDM communication systems constitute networks of certain topologies, where communication lines are interconnected at network nodes. Wavelength switching allows redirection of information between different WDM channels and provides the necessary flexibility and interconnectivity of the communication network. Currently, this function is implemented separately from the optical amplification system and, thus, adversely affects the cost of the communications system.
Significant transmission-rate limitations in current communication systems arise from pulse spreading due to group-velocity dispersion (GVD) in a single-mode optical fiber. Various passive dispersion-compensating devices have been proposed, including fiber-gratings, dual-mode fibers, dispersion-compensating fibers, etc. Integration of the functions of dispersion-compensation, broadband optical amplification and wavelength switching into a single device would bring significant advantages in reducing the overall complexity and cost of optical communication systems.
3. Discussion of Known Relevant Prior Art
The use of three-wave optical parametric amplification for optical signal amplification in fiber-optic communication systems was first described by Helmfrid et al, xe2x80x9cOptical Parametric Amplification of a 1.54 xcexcm Single-Mode DFB Laser in a Ti:LiNbO3 Waveguide, Journal of Lightwave Technology, Vol. 11, No. 9 (1993). This system did not achieve gain sufficient to be of practical use, although the publications predicts that with higher nonlinear coefficients or by using quasi-phase-matching, practical devices might be possible. Helmfrid et al also describe using their device to convert a signal wavelength to an idler wavelength. Yoo et al, in U.S. Pat. Nos. 5,825,517 and 5,434,700, demonstrate a device following the teachings of Helmfrid, using semiconductor quasi-phase matched material. This device had higher nonlinear coefficients and used quasi-phase matching yet did not produce significant optical signal amplification. The device is nevertheless useful in generating an idler frequency where the half-value of the pumping frequency falls between the idler and the signal frequencies (difference frequency generation). Yoo et al employ such a device as an optical interface between two WDM network loops. The later work by Yoo et al has applied difference frequency generation to WDM systems, including employing these devices as crossconnects between different WDM systems by adding optical switches.
M. H. Chou et al. (M. Fejer group), in xe2x80x9c1.5-xcexcm-band wavelength conversion based on difference-frequency generation in LiNbO3 waveguides with integrated coupling structuresxe2x80x9d, Optics Letters, Vol 23, No. 13 (1998) have disclosed difference frequency generation in waveguides of quasi-phase matched lithium niobate. The use of such devices for broadcasting a signal by converting one input wavelength to two different wavelengths has been demonstrated.
In spite of using higher nonlinearities and quasi-phase-matching, none of the aforementioned art has demonstrated amplification of the signal such that the resulting device can be used as a practical amplifier.
In this disclosure, the inventors describe how to obtain sufficient gain to make a practical OPA amplifier for telecommunications systems, as well as novel devices which can be implemented with the disclosed amplifier. Through proper design of the waveguide, a gain sufficient for optical telecommunications use may be obtained, while at the same time, the pumping requirements may be maintained sufficiently low, so as to be satisfied by readily available pump sources. A practical implementation of the use of a pulsed pump and intracavity pumping is also described. As used in this disclosure, the terms xe2x80x9csufficient gainxe2x80x9d, xe2x80x9cpractical amplifierxe2x80x9d and the like envision amplifier gains of at least around 3 dB.
The patents and publications discussed in the foregoing and hereinafter are hereby incorporated by reference herein.
It is a general object of the present invention to extend the number of WDM channels achievable in fiber-optic communication systems by enabling optical amplification at any wavelength within the complete spectral region of minimum optical loss of the optical fiber.
It is another object of the present invention to further increase the bit-rate of data transmission and to decrease the cost of communication systems by combining optical amplification and group-velocity dispersion compensation in the same device.
It is a still further object of the present invention to provide wavelength switching in WDM systems, which can be implemented in the same device as the optical amplification and GVD compensation.
Yet another object of the present invention is to provide optical data transmission at any wavelength within the complete spectral region of minimum loss in a fiber.
According to the present invention, these objects are achieved by employing three-wave optical parametric amplification in a nonlinear optical medium with a gain which is sufficiently high for fiber-optic communication systems, using acceptably low pump powers in the milliwatt to several watt range.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof.